Blood flow to the myocardium may be limited in patients with coronary artery disease secondary to stenosis of the coronary arteries from atherosclerosis. Blood flow limitation often occurs during periods of stress, and can lead to myocardial ischemia. Mechanisms of flow limitation may depend on the degree of stenosis or the type of stress (e.g. physical or mental) to which the heart is exposed. We examined myocardial blood flow at rest, and with physical and mental stress in a group of male and female patients with chronic coronary artery disease, ages 56-76. To measure blood flow, we used positron emission tomography (PET), with rubidium-82, a radioactive tracer that is injected intravenously and taken up by the heart in relation to blood flow. In 10 patients studied, 7 patients showed an attenuated increase in blood flow in areas served by diseased arteries with exercise compared to normal regions of the heart in which flow increased to a greater extent. However, 3 patients showed a decrease in blood flow in the abnormal regions, Suggesting that constriction of the diseased vessel may be a factor in these patients. These patients had more abnormal stress treadmill tests, with ischemia occurring at lower workloads than those patients without an absolute decrease in flow. This phenomenon was not observed with mental stress. Identification of patients with this vasoconstrictor response may have prognostic and therapeutic implications.